Supported metal catalysts possessing hydrogenation-dehydrogenation functionality have found many applications in petroleum refining processes. In these catalysts, a metal or metal compound which provides hydrogenation-dehydrogenation function is supported on a porous, inorganic oxide support such as alumina, silica, silica-alumina or a crystalline material with defined porosity characteristics such as one of the aluminosilicate zeolites. The support may itself possess catalytic activity e.g. acidic (cracking) activity so that the catalyst as a whole is bifunctional. Typical metals used in these catalysts include noble metals such as platinum, rhenium, iridium and palladium and base metals, especially those of Groups VIA and VIIIA of the Periodic Table (IUPAC Table), especially nickel, cobalt, molybdenum, tungsten and vanadium. Catalysts of this type are conventionally used in petroleum refining and petrochemical processes such as reforming, hydroprocessing e.g. hydrotreating, hydrofinishing, hydrocracking, isomerisation and dewaxing.
In many of these processes, the catalyst becomes deactivated during use because coke (a highly carbon-rich hydrocarbon) becomes deposited on the catalytic sites which then are no longer accessible to the reacting species. Deactivation may also ensue from agglomeration of the metal component under severe conversion conditions especially high temperature or by deposition by poisons. In order to restore catalytic activity and selectivity, the coke is removed either periodically or continuously, by oxidative regeneration; the coke-containing catalyst is exposed to a stream of oxygen-containing gas, usually air, which burns the coke off the support. At the same time, many poisons are driven off under the high temperatures which prevail during the regeneration. Oxidative regeneration techniques are widely known and are described, for example, in U.S. Pat. Nos. 3,069,362; 3,069,363 and British Pat. No. 1,148,545.
Another restorative technique is hydrogen reactivation, which is commonly employed between oxidative regenerations to remove accumulated coke or adsorbed material which can lower catalyst activity. Under the conditions employed in treatments of this kind, the hydrogen reacts with the coke to form hydrogen-enriched compounds which are more mobile and which are removed from the catalyst while adsorbed catalyst poisons are removed by the stripping action of the hydrogen. The hydrogen may be used as such or mixed with inert gases or gas mixture such as nitrogen, methane, carbon dioxide, carbon monoxide or flue gas as described, for instance, in U.S. Pat. Nos. 4,358,395 and 4,508,836. Hydrogen treatment may also precede an oxidative regeneration treatment.
One problem which is commonly encountered with these restorative treatments is metal agglomeration. This problem, which is particularly severe with the catalysts containing mobile metals, especially platinum and palladium, arises from the use of the high temperatures conventionally associated with oxidative regeneration and with certain hydrogen treatments. Metal agglomeration may also occur if high temperatures are encountered during the actual proessing step before the regeneration. When agglomeration of the metal component occurs, the particles of the metal component which originally are present in a highly dispersed state of the catalyst coalesce into perceptibly larger particles. As a result of this phenomenon, the catalyst tends to lose activity and selectivity because many of the reactions requiring bifunctional catalysis rely upon the proximity of the two types of catalytic site for the appropriate mechanistic steps to proceed.
Processes for redispersing metal components on the support are known and are generally referred to as rejuvenation processes. They are commonly used for reforming catalysts which encounter high temperatures during the endothermic reforming process. Typically, these rejuvenative techinques employ a halogen to redistribute the noble metal component which tends to agglomerate at the high temperatures associated with reforming and oxidative regeneration. Examples of rejuvenative processes may be found in U.S. Pat. Nos. 2,906,702; 3,134,732 and 3,986,982. Reference is also made to "Catalyst Deactivation and Regeneration:, Chemical Engineering, 91, No. 23, 12 November 1984. Another typical rejuvenative process is described in U.S. Pat. No. 3,134,732 (Kearby): a coked platinum catalyst on an alumina support is oxidatively regenerated and then contacted with gaseous halogen at a maximum temperature of 1250.degree. F. (675.degree. C.) to reduce the crystallite size of the platinum.
Rejuvenation processes such as these may be regarded as at best remedial: they attempt to alleviate the problem only after it has arisen. An alternative approach is prophylactic, to prevent the problem arising by improving the stability of the dispersed metal component so that it does not agglomerate. It is known that the interaction between the metal component and the substrate will affect the bonding of the metal crystallites. For example, noble metals sinter onto silica supports more strongly than onto alumina supports as discussed in J. Catalysis 55, 348-360 (1978) and AIChE Paper "Sintering/Redispersion in Supported Metal Catalysts: Phenomena and Analyses," Dadyburjor et al. AIChE 1983. See also S. J. Tauster: "Strong Metal-Support Interactions," Ed. by R. T. Baker, S. J. Tauster, and J. A. Dumesic, ACS Symposium Series 298, American Chemical Society, Washington, D.C., 1986, Chapter 1, p. 1.
The present invention is directed to a technique for improving the dispersability of metal components on zeolitic catalyst supports by modifying the composition of the support.